1. Field of the Invention
The present invention relates to an electronic component that includes a capacitor provided on a substrate, for example formed by semiconductor processing technology.
2. Description of the Related Art
In a radio frequency (hereinafter, RF) system such as a mobile phone or a wireless LAN, signals have to be subjected to phase-matching for satisfactory transmission among functional devices constituting the system. Accordingly, the input/output terminal of each device is generally provided with a passive component such as an inductor or a capacitor, and that acts as a phase shifter for controlling the phase of the signals.
In the RF system in general, a SAW filter is employed for use as a narrow-band frequency filter. The SAW filter, which includes a piezoelectric element, produces a difference in potential between piezoelectric element electrodes because of a piezoelectric effect, when a physical impact or a thermal effect is applied to the SAW filter or the piezoelectric element thereof during the manufacturing process of the apparatus in which the SAW filter is incorporated. In this case, a predetermined voltage is applied to an electronic component electrically connected to the SAW filter. The capacitor included in the passive element (phase shifter) is usually electrically connected to the SAW filter, and hence the capacitor has to have a high withstand voltage (150 V or higher, for instance), to thereby prevent a dielectric breakdown between the capacitor electrodes, which may take place upon application of a voltage accidentally generated by the SAW filter or the piezoelectric element thereof.
There has been a constant demand for reduction in dimensions of various parts composing the RF system, driven by the increase in number of parts for achieving a higher performance. For making the system smaller in dimensions, an integrated passive device (hereinafter, IPD) manufactured based on a semiconductor processing technology, which includes a plurality of predetermined passive components such as an inductor, a capacitor, a resistance and a filter densely integrated on a substrate, may be employed as the passive element (phase shifter). When employing the IPD, the capacitor included therein still has to have a high withstand voltage, for preventing a dielectric breakdown between the capacitor electrodes, as stated above. Techniques related to the IPD are found, for example, in JP-A-H04-61264 and JP-A-2002-33239.
The capacitor included in the IPD generally has a laminate structure consisting of a lower electrode layer provided on a substrate, an upper electrode layer opposed to the lower electrode layer, and a dielectric layer disposed between the electrode layers. The lower electrode layer and the upper electrode layer are both expected to have low resistance and to achieve sufficient adhesion to the dielectric layer. Insufficient adhesion between the layers provokes deterioration with time in adhesion status between those layers (for example, expansion of a gap between the layers with time), thus causing variation in static capacitance characteristic of the capacitor, which is undesirable.
Conventionally, the lower electrode layer of the capacitor included in the IPD is often constituted of a multilayer structure including Ti/Au/Ni/Au layers. Specifically, such multilayer structure includes a Ti layer (for instance, 50 nm in thickness) provided on a substrate, a first Au layer (for instance, 500 nm in thickness) provided on the Ti layer, a Ni layer (for instance, 50 nm in thickness) provided on the first Au layer, and a second Au layer (for instance, 500 nm in thickness) provided on the Ni layer, such that a minute amount of Ni originating from the Ni layer reaches the surface of the second Au layer on the side of the dielectric layer, because of thermal diffusion. Such multilayer structure is known to have low resistance, and to achieve sufficient adhesion especially to a SiO2 dielectric layer, because of the presence of Ni on the surface of the second Au layer on the side of the dielectric layer.
Employing the Ti/Au/Ni/Au multilayer structure as the lower electrode layer, however, may not always assure the required adhesion performance between the lower electrode layer and the dielectric layer. Also, employing the Ti/Au/Ni/Au multilayer structure as the lower electrode layer may lead to failure in attaining the withstand voltage of a required level in the capacitor. Increasing the thickness of the dielectric layer may help in improving the withstand voltage of the capacitor, however the increase in thickness of the dielectric layer requires as much increase in area of the upper electrode layer, in order to secure the required static capacitance of the capacitor. Therefore, increasing the thickness of the dielectric layer is not a desirable solution, from the view point of suppressing an increase in dimensions of the capacitor, hence the IPD including the capacitor as a component.